Acoustic transducer including piezoelectric driving element

ABSTRACT

A disk shaped piezoelectric element constructed to operate in a planar mode so as to define a first overtone nodal ring on one of the major surfaces, a conically shaped diaphragm having a truncated apex defining a generally circular area affixed to a major surface of the element concentric with the nodal ring and spaced radially therefrom so as to reduce the amplitude of the output of the first overtone to approximately the amplitude of the output of the fundamental frequency, and a rubber disk affixed to the opposite major surface of the piezoelectric element to lower the fundamental resonance frequency and damp the peak output of the fundamental and first overtone resonance frequencies to provide a flat response over a desired bandwidth.

United stais' chafft 1 ACOUSTIC TRANSDUCER INCLUDING PIEZOELECTRICDRIVING ELEMENT [75] Inventor: Hugo W. Schafft, Des Plaines, I11.

[73] Assignee: Motorola, Inc., Franklin Park, 111.

[22] Filed: Apr. 10, 1972 [21] Appl. No.: 242,501

151 Jan. 15, 1974 Primary Examiner-William C. Cooper AssistantExaminer-Douglas W. Olms romqx: Eug r eni [5 7 ABSTRACT A disk shapedpiezoelectric element constructed to operate in a planar mode so as todefine a first overtone nodal ring on one of the major surfaces, aconically shaped diaphragm having a truncated apex defining a generallycircular area affixed to a major surface of the element concentric withthe nodal ring and spaced radially therefrom so as to reduce theamplitude of the output of the first overtone to approximately theamplitude of the output of the fundamental frequency, and a rubber diskaffixed to the opposite major surface of the piezoelectric element tolower the fundamental resonance frequency and damp the peak output ofthe fundamental and first overtone resonance frequencies to provide aflat response over a desired bandwidth.

15 Claims, 5 Drawing Figures [52] US. Cl. 179/110 A, 181/32 R, 310/83[51] Int. Cl H04r 17/00 [58] Field of Search 179/110 A; 181/32 R, 181/32A; SIG/8.6, 8.3

[56] References Cited UNITED STATES PATENTS 2,518,331 8/1950 Kalin179/110 A 3,588,381 6/1971 Schafft 1 SIG/8.6 3,548,116 12/1970 .Schafft179/110 A 3,654,402 4/1972 Roos 179/110 A 3,698,993 10/1972 Rauh 181/32R UR Im 179/11011 ACOUSTIC TRANSDUCER INCLUDING PIEZOELECTRIC DRIVINGELEMENT BACKGROUND OF THE INVENTION 1. Field of the Invention Acoustictransducers are utilized to convert electrical energy to sound oracoustic energy to electrical energy. In the present embodiment apiezolectric element is utilized for a driver, which piezoelectricelement bends or warps in a particular mode in response to electricalenergy being applied thereacross or produces electrical energythereacross in response to bending or warping thereof. While the presentinvention might be utilized in acoustic transducers for convertingmechanical energy to electrical energy, it is especially useful inacoustic transducers, such as speakers and the like, convertingelectrical energy to acoustic energy or sound. In the speaker art, it ishighly desirable to provide a transducer with a flat response over adesired bandwidth, i.e., all frequencies of sound between two desiredfrequencies are produced at approximately equal amplitudes. Becausepiezoelectric elements are mechanical vibrating devices, they havespecific resonant frequency points, referred to as the fundamental,first overtone, second overtone, etc., at which points the amplitude ofthe output is substantially increased.

2. Description of the Prior Art In prior art acoustic transducers, anyflattening of the frequency response is accomplished through design ofSUMMARY OF THE INVENTION The present invention pertains to apparatusproviding conversion between electrical and mechanical stimuli includinga piezoelectric element constructed to operate in a planar mode andhaving first overtone nodal lines present on a major surface, agenerally conically shaped diaphragm with truncated apex defining acircular area affixed to the major surface of the element approximatelycentrally within the first overtone nodal line so as to be spaced fromthe line sufficiently to reduce the amplitude of the output of the firstovertone to approximately the amplitude of the output of thefundamental, and a resilient damping member affixed to an opposed majorsurface of the piezoelectric element to lower the fundamental frequencyof the element and damp the fundamental and first overtone peaks.

It is an object of the present invention to provide an improvedpiezoelectric driven acoustic transducer.

It is a further object of the present invention to provide apiezoelectric transducer having a substantially flat response over adesired bandwidth These and other objects of this invention will becomeapparent to those skilled in the art upon consideration of theaccompanying specification, claims and drawmgs.

7 output peak in the response at approximately 1 KC,

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings, whereinlike characters indicate like parts throughout the figures:

FIG. 1 is an axial sectional view of an acoustic transducer constructedin accordance with the present invention;

FIG. 2 is a view (a) in top plan of a piezoelectric driver illustratingnodal rings, (b) in side elevation of the driver illustrating thefundamental nodal ring, (0) in side elevation of the driver illustratingfirst overtone nodal rings, and (d) the truncated apex of a diaphragm;

FIG. 3 is a graph illustrating generally the frequency response of atransducer similar to that illustrated in FIG. 1 constructed inaccordance with prior art techniques and, in dotted lines, the frequencyresponse of a transducer similar to that illustrated in FIG. 1constructed in accordance with the present invention;

FIG. 4 is an enlarged sectional view of a piezoelectric driving elementand a resilient damping member affixed thereto, portions thereofremoved; and

FIG. 5 is a greatly enlarged fragmentary view of the resilient dampingmember.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the figures the numeral 10generally designates a piezoelectric transducer utilized for providingconversions between electrical and mechanical stimuli. Applications fortransducers of this type are speakers, sound sensors, etc. Thetransducer 10 includes a housing 11 defining a generally cup shapedcavity l2, a generally conically shaped diaphragm l3 affixed within thecavity 12 by its outermost edges and having a truncated apex to which isattached a piezoelectric driver 15 with a piezoelectric element 16 anddamping member 17. The operation of piezoelectric transducers wherein apiezoelectric driver is connected directly to a diaphragm and solelysupported thereby is described in detail in my U. S. Pat. No. 3,548,116,entitled Acoustic Transducer Including Piezoelectric Wafer SolelySupported By A Diaphragm, 1970.

In the prior art structure of the above described patent the apex of theconically shaped diaphragm is fixedly attached to the center of apiezoelectric element and the frequency response of the transducer isapproximated by the solid line graph in FIG. 3. The frequenciesspecified in FIG. 3 are exemplary and may vary somewhat with variationsin transducers. A first designated 20, is produced primarily by a centersupported resonance of the driver at 1 KC (a resonance caused bymounting the driver at the center with a relatively stiff diaphragm)which is coupled to the diaphragm by a slight axial movement of thediaphragm and driver, at the point of connection therebetween, whichaxial movement is present because the diaphragm is not rigid. A secondoutput peak at approximately 5 KC, designated 21, is produced primarilyby the fundamental resonance of the piezoelectric element 16. A thirdoutput peak at approximately l9 KC, designated 22, is produced primarilyby the first overtone resonance of the piezoelectric element 16.

Referring to FIG. 2, the piezoelectric element 16 is illustrated in topplan in (a) and in side elevation in (b) and (c). The piezoelectricelement 16, as illustrated in and issued Dec. 15,-

FIG. 2, is disk shaped, but it should be understood that substantiallyany flat shape might be utilized wherein the element operates in aplanar bending mode, i.e., flexing or distorting along more than oneaxis. For example, the element 16 might be square or even irregularlyshaped. However, in the present embodiment and to simplify theexplanation, a disk shaped element 16 will be described and theoperation thereof explained.

At the fundamental resonance frequency, the element 16 vibrates alongeach diameter thereof as illustrated in FIG. 2b. During the first halfcycle of eachvibration the center of the element 16 flexes upwardlywhile the end portions turn downwardly as indicated by the dotted line25 and during the second half cycle of each vibration the reverseoccurs, as illustrated by the dotted line 26. It will be noted that twonodes are formed along the diameter, at which points no axial movementof the element 16 occurs. Because every diameter of the element 16reacts, at the fundamental resonance frequency, as illustrated in FIG.2b, the nodes define a nodal ring 27 on the upper and lower majorsurfaces of the element 16, which ring 27 is a locus of nodes or pointshaving no axial movement at the fundamental resonance frequency.

In a similar fashion FIG. illustrates movement of the element 16 at thefirst overtone resonance frequency. Because of the first overtone beinga higher frequency, two concentric nodal rings 28 and 29 are defined onthe major surfaces of the element 16. The nodal ring 29 is concentricwith the nodal ring 27 defined by the fundamental frequency and spacedradially inwardly therefrom. It will be noted from a comparison of FIG.2b and c that the output, or amount of axial movement, of the element 16at the fundamental resonance frequency is substantially constantthroughout the area encircled by the nodal ring 29. Further, since theamount of movement of the element 15 determines the amount of movementof the diaphragm l3, and hence, the output or conversion betweenstimuli), connecting the diaphragm at a point concentric to the nodalrings 27, 28 and 29, as in the prior art, will provide the maximumoutput of all frequencies and result in the frequency responseillustrated in full lines in FIG. 3.

In FIG. 2d a portion of the diaphragm 13 is illustrated with the apexthereof truncated to define a generally circular area having a diametersmaller than the diameter of the nodal ring 29. If the diameter of thetruncated apex of diaphragm 13 is equal to the diameter of the nodalring 29 the first overtone will be substantially eliminated, since theoutput or axial movement of the element 16 at the nodal ring 29 for thefirst overtone is zero. Thus, by forming the diaphragm 13 so that thetruncated apex has a diameter greater than a point and less than thediameter of the nodal ring 29, the output of the first overtone (peak 22in FIG. 3) can be diminished to approximately the amplitude of thefundamental (peak 21 in FIG. 3). Since the second overtone issubstantially above the first overtone and beyond the desired frequencyresponse in acoustic transducers, it is not necessary to considerovertones beyond the first.

Referring more specifically to FIG. 4, the piezoelectric driver 15 isillustrated in enlarged cross section, with a portion thereof removed.The piezoelectric element 16 includes first and second piezoelectricwafers 40 and 41 having an electrode 42 sandwiched therebetween andelectrodes 43 and 44 fixedly engaged in overlying relationship onopposed major surfaces thereof. The operation of the element 16 is wellknown to those skilled in the art and, as previously mentioned, isdescribed in detail in U. S. Pat. No. 3,548,l 16. It is, therefore,sufficient to state at this time that the electrodes 42, 43 and 44 drivethe element 16 in a planar mode of operation. The element 16 hasresilient damping member 17 affixed to the major surface thereof opposite the major surface having the diaphragm l3 affixed thereto. In thepresent embodiment, since the element 16 is generally disk shaped, thedamping member 17 is also disk shaped, and as illustrated, has aslightly larger diameter than the element 16. The damping member 17damps or loads the movement of the element 16 to reduce the fundamentalpeak 21 and to further reduce the first overtone peak 22 so that thefrequency response of the driver 15 approaches the dotted line curve 50in FIG. 3.

The damping member 17 is formed of a resilient material, such as rubber(the term rubber being understood to include natural and syntheticmaterials) or other elastomers. Elastomers generally have a frequencydependent shear modulus which varies directly with the frequency ofstresses applied thereto, i.e., the shear modulus increases with thefrequency of stresses applied thereto. At static or slowly occurringstresses the elastomeric material operates in a rubbery region in whichit appears elastic to the forces operating upon it. However, as higherfrequency dynamic stresses are applied the shear modulus is increasedand the elastomer goes through a glassy transition region into a glassyregion where it appears metallic or hard. At the lower frequencies,around the peaks 20 and 21 of FIG. 3, the damping member 45 ispreferably operating in the glassy transition region and introduceshysteresis losses which substantially remove the peaks 20 and 21.However, at the upper frequencies, around the peak 22, the material ofthe damping member 17 may begin to approach the glassy region and thehysteresis losses are substantially reduced. Thus, the damping effect ofthe member 17 at the peak 22 is greatly reduced.

To compensate for the reduction in hysteresis losses small particles 46of a relatively heavy material, such as iron or lead, are intermixedwith the resilient material of the clamping member 17 during theformation of the disk. These particles 46 add additional weight to themember 17 and introduce a coulomb type damping which is caused byinternal friction between the metal particles and their enclosing rubberwalls. This internal friction is caused by a relative movement due to adifference in inertia of the heavy particles and the surroundingresilient material of the clamping member 17. This frictional or coulombtype damping increases with the number of particles and the size of theparticles. It has been found, for example, that lead particles havingapproximately a I00 mesh size mixed with rubber in a 3 to 1 ratio, byweight, provide a desired amount of damping for the frequency responseillustrated in FIG. 3. Small amounts of a lubricant, such as graphite,may also be added to the damping member 17, as illustrated in FIG. 5 bythe numeral 47, to increase the relative movement between the heavyparticles 46 and the elastomeric or rubber material and, therefore,increase the damping action.

The effect of the reduction in hysteresis losses at the higherfrequencies can also be reduced or eliminated by selecting anelastomeric material with a glass transition region above the highestfrequency desired for the frequency response of the transducer 10. Ithas been found for example that neoprene has a relatively high glasstransition region and may, in many instances, provide sufficient dampingat high frequencies so that the addition of particles 46 is notrequired. It should be understood that the type of materials utilizedand the frequency response desired dictate the ultimate construction ofthe transducer 10.

In addition to providing the damping function described above, thedamping member 17 increases the mass of the driver 15 and, therefore,improves the weight ratio of the driver 15 over the diaphragm 13. Thisimproved weight ratio results in a tighter coupling between the driver15 and the diaphragm 13 at the lower frequencies. Thus, in someinstances, although the material selected for the damping member 17provides sufficient damping at the higher frequencies, it may bedesirable to add relatively heavy particles 46 to the member 17 toincrease the mass of the driver 15.

The addition of the clamping member 17 to the driver 15 (and thediaphragm 13 to a much smaller extent) lower the fundamental resonancefrequency of the driver 15. Referring to FIG. 3, it can be seen that theknee 51 of the flattened response curve 50 (illustrated in dotted lines)occurs slightly below the peak 21 for the fundamental resonancefrequency. The diameter and thickness of the damping member 17 should beadjusted to lower the resonance frequency of the combined piezoelectricelement 16 and damping member .17 to a point below the fundamentalresonance frequency of the piezoelectric element 16 (peak 21) such thatthe curve 50 falls away sharply at the lower frequencies (as illustratedin FIG. 3). If the resonance fre quency of the driver 15 is too high itwill add to the peak 21 and produce too high an output at the lowerfrequencies while not extending the frequency response sufficiently intothe lower frequency range. If the resonance frequency of the driver 15is lowered too much the curve 50 will not fall away sharply at the lowerfrequencies but will rise at a much lower rate. Thus, through carefulselection of the diameter thickness and mass of the clamping member 17the desired flat response of the transducer can be extended somewhatinto the lower frequency range. Further, through careful selection ofthe type and thickness of material and the amount and particle size ofparticles 46 the degree of damping can be adjusted to provide asubstantially flat response over a desired band of frequencies. Itshould be noted that the damping member 17 might be constructed with anannular configuration, in which case high frequency damping iscontrolled by adjusting the size of the inside diameter, since highfrequency damping is most effective in the center of the driver 15.

The cavity 12 in the housing 11 has a resonant frequency which, in manyinstances appears in the desired frequency response of the transducer10. At the cavity resonance there is a tendency to absorb output powerfrom the transducer 10 and, thus, a notch (not shown) will appear in theoutput curve 50 of HO. 3. To prevent the power output loss and theresulting distortions, acoustic absorbing material, in the presentembodiment an annularly shaped member 55 of foam rubber or the like, isplaced in the cavity 12 between the housing 11 and the diaphragm 13. Themember 55 alters the cavity resonance, or lowers the Q of the cavity 12,to

substantially eliminate the absorbing of power and consequent notch inthe output curve 50. It should be understood that the acoustic absorbingmaterial is only utilized when the cavity resonance falls within thedesired frequency response and, in some instances, it may be possible toeliminate the material through design of the transducer components.

Thus, an improved piezoelectric transducer is disclosed having an outputwhich is substantially flat throughout a desired band of frequencies.The transducer output has been referred to throughout the specificationand it should be understood that this refers to either mechanical orelectrical output in response to either electrical or mechanical input,respectively. Further, in addition to a flat response and improvedcoupling at the lower frequencies, the critically damped element 16 hasan improved transient response over known electrodynamic drive systemsfor transducers. While I have shown and described a specific embodimentof this invention, further modifications and improvements will occur tothose skilled in the art. I desire it to be understood, therefore, thatthis invention is not limited to the particular form shown and I intendin the appended claims to cover all modifications which do not departfrom the spirit and scope of this invention.

I claim:

1. Apparatus providing conversions between electrical and mechanicalstimuli comprising:

a. a piezoelectric element having a generally flat major surface,electrodes attached to said element for driving said element in a planarbending mode when said electrodes are properly energized, and firstovertone nodal lines present on said major surface during planar bendingmode operation of said element;

b. a generally conically shaped diaphragm having a truncated apexdefining a generally circular area with a diameter larger than a pointand sufficiently less than the distance between nodes of the firstovertone to reduce the amplitude of the output of the first overtone toapproximately the amplitude of the output of the fundamental; and

c. means fixedly attaching the truncated apex of said diaphragm to saidpiezoelectric element with the circular area defined by said apexgenerally encircled by and substantially centered within the firstovertone nodal lines.

2. Apparatus as set forth in claim 1 wherein the piezoelectric elementis generally disk shaped and the nodal line for the first overtonedefines a generally circular area concentric with the truncated apex ofthe diaphragm.

3. Apparatus as set forth in claim 1 wherein the piezoelectric elementincludes two piezoelectric wafers affixed together in parallelcontiguous relationship with electrodes on each side of each wafer.

4. Apparatus as set forth in claim 1 having in addition a resilientclamping member affixed to the piezoelectric element on the sideopposite the diaphragm for lowering the fundamental resonance frequencyand damping the resonance peak thereof to extend the range of thespeaker to lower frequencies and to provide a relative flat responseover the entire range.

5. Apparatus as set forth in claim 4 wherein the damping member isformed from a material including rubber.

6. Apparatus as set forth in claim wherein the rubber has a glasstransition region approximately including the frequency of the firstovertone.

7. Apparatus as set forth in claim 5 wherein the rubber includesneoprene.

8. Apparatus as set forth in claim 5 wherein the clamping memberincludes particles of a relatively heavy material intermixed with therubber for providing frictional damping at the higher frequencies ofoperation.

9. Apparatus as set forth in claim 8 wherein the particles include leadof approximately 100 mesh size and three parts by weight of lead to onepart by weight of rubber.

10. Apparatus as set forth in. claim 8 wherein the damping memberfurther includes particles of dry lubricant intermixed with theparticles of relatively heavy material for increasing relative motionbetween the particles of relatively heavy material and the rubber at thehigher frequencies of operation.

11. Apparatus as set forth in claim 4 wherein the damping member extendsoutwardly beyond the edges of the piezoelectric element for furtherdamping the lower frequencies of operation.

12. Apparatus as set forth in claim 4 wherein the combined mass of thepiezoelectric element and the damping member is substantially heavierthan the mass of the diaphragm.

13. An improved acoustic transducer comprising:

a. a housing defining a cavity therein;

b. a generally disk shaped piezoelectric element having opposedgenerally flat major surfaces and defining thereon a nodal ring for afirst overtone frequency, said element having electrodes attachedthereto for driving said element in a planar bending mode when saidelectrodes are properly energized;

c. a generally conically shaped diaphragm having a truncated apexdefining a generally circular area with a diameter larger than a pointand sufficiently different from the diameter of the first overtone nodalring to reduce the amplitude of the output of the first overtone toapproximately the amplitude of the output of the fundamental;

d. means fixedly attaching the truncated apex of said diaphragm to oneof said major surfaces of said piezoelectric element with the circulararea defined by said apex generally concentric with the first overtonenodal ring; and

e. means operatively mounting said diaphragm within the cavity of saidhousing.

14. An improved acoustic transducer as set forth in claim 13 having inaddition a generally disk shaped resilient damping member affixed to themajor surface of said piezo-electric element opposite the major surfacehaving the diaphragm affixed thereto.

15. An improved acoustic transducer as set forth in claim 14 having inaddition acoustic absorbing material positioned in the cavity of saidhousing generally between said housing and said diaphragm.

1. Apparatus providing conversions between electrical and mechanicalstimuli comprising: a. a piezoelectric element having a generally flatmajor surface, electrodes attached to said element for driving saidelement in A planar bending mode when said electrodes are properlyenergized, and first overtone nodal lines present on said major surfaceduring planar bending mode operation of said element; b. a generallyconically shaped diaphragm having a truncated apex defining a generallycircular area with a diameter larger than a point and sufficiently lessthan the distance between nodes of the first overtone to reduce theamplitude of the output of the first overtone to approximately theamplitude of the output of the fundamental; and c. means fixedlyattaching the truncated apex of said diaphragm to said piezoelectricelement with the circular area defined by said apex generally encircledby and substantially centered within the first overtone nodal lines. 2.Apparatus as set forth in claim 1 wherein the piezoelectric element isgenerally disk shaped and the nodal line for the first overtone definesa generally circular area concentric with the truncated apex of thediaphragm.
 3. Apparatus as set forth in claim 1 wherein thepiezoelectric element includes two piezoelectric wafers affixed togetherin parallel contiguous relationship with electrodes on each side of eachwafer.
 4. Apparatus as set forth in claim 1 having in addition aresilient damping member affixed to the piezoelectric element on theside opposite the diaphragm for lowering the fundamental resonancefrequency and damping the resonance peak thereof to extend the range ofthe speaker to lower frequencies and to provide a relative flat responseover the entire range.
 5. Apparatus as set forth in claim 4 wherein thedamping member is formed from a material including rubber.
 6. Apparatusas set forth in claim 5 wherein the rubber has a glass transition regionapproximately including the frequency of the first overtone. 7.Apparatus as set forth in claim 5 wherein the rubber includes neoprene.8. Apparatus as set forth in claim 5 wherein the damping member includesparticles of a relatively heavy material intermixed with the rubber forproviding frictional damping at the higher frequencies of operation. 9.Apparatus as set forth in claim 8 wherein the particles include lead ofapproximately 100 mesh size and three parts by weight of lead to onepart by weight of rubber.
 10. Apparatus as set forth in claim 8 whereinthe damping member further includes particles of dry lubricantintermixed with the particles of relatively heavy material forincreasing relative motion between the particles of relatively heavymaterial and the rubber at the higher frequencies of operation. 11.Apparatus as set forth in claim 4 wherein the damping member extendsoutwardly beyond the edges of the piezoelectric element for furtherdamping the lower frequencies of operation.
 12. Apparatus as set forthin claim 4 wherein the combined mass of the piezoelectric element andthe damping member is substantially heavier than the mass of thediaphragm.
 13. An improved acoustic transducer comprising: a. a housingdefining a cavity therein; b. a generally disk shaped piezoelectricelement having opposed generally flat major surfaces and definingthereon a nodal ring for a first overtone frequency, said element havingelectrodes attached thereto for driving said element in a planar bendingmode when said electrodes are properly energized; c. a generallyconically shaped diaphragm having a truncated apex defining a generallycircular area with a diameter larger than a point and sufficientlydifferent from the diameter of the first overtone nodal ring to reducethe amplitude of the output of the first overtone to approximately theamplitude of the output of the fundamental; d. means fixedly attachingthe truncated apex of said diaphragm to one of said major surfaces ofsaid piezoelectric element with the circular area defined by said apexgenerally concentric with the first overtone nodal ring; and e. meansoperatively mounting said diaphragm within the cavity of said housing.14. An improved acOustic transducer as set forth in claim 13 having inaddition a generally disk shaped resilient damping member affixed to themajor surface of said piezoelectric element opposite the major surfacehaving the diaphragm affixed thereto.
 15. An improved acoustictransducer as set forth in claim 14 having in addition acousticabsorbing material positioned in the cavity of said housing generallybetween said housing and said diaphragm.